Now I don't know if it's intentional or not, but many things you are saying are just FUD. You seem to be assuming that the only way to do JVM stuff is considering all the difficulties that enterprise Java shops with legacy projects put up with. That is
far from the truth, as any tutorial for Spray, Play or Akka could tell you.
> Which JDK do I use[1]?
The Oracle JDK (which is the same as OpenJDK FYI).
> What happens when one library I want to use recommends a different JDK than another library I want to use?
There is a 3-month period or so every three years when a new major Java release is done when some developers of enterprisy libraries might want to do some more testing before making a recommendation. It's not a big question in the JVM world at all, and it's definitely not a big problem even for early adopters.
> Build tools questions: ant, maven, gradle?
SBT. You're building a new Scala project.
> most of it is required-but-incidental; none of it (except Scala, in our example) is an inherently necessary part of getting from learning to a production system
You would have been right five years ago. Today, many people run Akka/Spray/Play projects in production just fine without deeper devops knowledge necessary than with other runtimes.